An infant had a giant congenital nevus, neurocutaneous melanosis (NCM), and a Dandy-Walker malformation of the brain. The diagnosis of NCM was suspected at 6 weeks of age when macrocephaly was noted, resulting in the discovery of hydrocephalus and a Dandy-Walker malformation. Serial magnetic resonance imaging scans demonstrated so-called T1 shortening in the pia or subarachnoid spaces surrounding the cerebellar vermis and in the temporal lobes anterior to the temporal horns. Eventually, a biopsy-proved melanoma developed in the anterior temporal lobe, in an area previously noted to have T1 shortening. Since meningeal cells have been shown experimentally to play a critical role in cerebellar development, we hypothesize that the association of NCM with a Dandy-Walker malformation may be due to meningeal melanosis disrupting the normal development of the cerebellum and fourth ventricle.